Effect of molecular weight on the refractive index increment of polystyrenes in solution

Abstract

The variation of refractive index increment dn dc with molecular weight has been studied using solutions of monodisperse polystyrenes (2000< M w <4×10 6) in benzene. The results are in good agreement with those obtained by several authors using other systems. We have shown that the linear relationship: dn dc = dn dc m+ K″ Mn where ( dn dc) m is the refractive index increment of the repeating unit and K″ a constant, holds only for low molecular weights. With the assumption of volume additivity, we have estimated quantitatively the linear portion of the curve, dn dc =f( 1 M ) , observed for molecular weights below 20 000 as a function of polymer composition. Beyond this limit, the increase of dn dc with molecular weight may be probably related to segment-segment interactions within the coil.